The invention concerns a method for calibrating an exhaust gas sensor and a device for calibrating an exhaust gas sensor.
To control the operation of internal combustion engines, exhaust gas sensors are used which supply measurement signals to the engine control system and/or to an exhaust gas aftertreatment system. The exhaust gas sensors used are primarily lambda sensors and NOx sensors. To be able to perform an effective catalytic reduction, for example by the controlled addition of urea, it is necessary to determine the NOx emissions in the exhaust gas flow from an internal combustion engine using NOx sensors. DE 101 00 420 A1 describes a method for controlling an exhaust gas aftertreatment system in which a predefinable quantity of reducing agent is supplied to the exhaust gas flow depending on the state of the internal combustion engine and/or the exhaust gas aftertreatment system.
To measure the oxygen content in the exhaust gas flow from an internal combustion engine, lambda sensors are used which are inserted as exhaust gas sensors in the exhaust system before and/or after a catalytic converter.
The exhaust gas sensors used are subject to an ageing process which means that during a lengthy operating period of an exhaust gas sensor, the measurement curve changes as a function of the operating period. To undertake calibration of a lambda sensor, in vehicle engines it is known in principle to calibrate the lambda sensor in overrun mode in which no exhaust gas is generated and the lambda sensor is exposed to the aspirated ambient air. Provided that the aspirated ambient air has an oxygen content of 20.942%, a measurement value output by the lambda sensor can be corrected accordingly if, because of a lengthy operating period, the lambda sensor is supplying a measurement value deviating from the actual value. Such a measurement value adaptation can be carried out for example by applying a correction factor to the measurement value output by the lambda sensor.
DE 10 2008 046 121 A1 discloses a method for calibrating an exhaust gas sensor which is arranged in an exhaust pipe of an internal combustion engine and protrudes laterally into an exhaust channel. To calibrate the exhaust gas sensor, its measurement tip is exposed to a passing flow of flushing air. This document also mentions the possibility that a screening device can be guided at the measurement tip in order to conduct the flushing air to the measurement tip in a targeted fashion. In normal operating mode, the screening device is removed from the exhaust pipe.